3.1-release-note-draft.md

Features

Migration Build
compiler-core: whitespace handling strategy (dee3d6a)
support component-level compilerOptions when using runtime compiler (ce0bbe0)
config: support configuring runtime compiler via app.config.compilerOptions (091e6d6)
support casting plain element to component via is="vue:xxx" (af9e699)
devtools: improved KeepAlive support (03ae300)
devtools: performance events (f7c54ca)
onServerPrefetch (#3070) (349eb0f)

Performance Improvements

only trigger $attrs update when it has actually changed (5566d39)
compiler: skip unncessary checks when parsing end tag (048ac29)
avoid deopt for props/emits normalization when global mixins are used (51d2be2)

Deprecations

app.config.isCustomElement has been deprecated and should be now nested under app.config.compilerOptions. [Docs]
delimiters component option has been deprecated and should now be nested under the compilerOptions component option. [Docs]
v-is has been deprecated in favor of is="vue:xxx" [Docs]

Minor Breaking Changes

this.$props and the props object passed to setup() now always contain all the keys for declared props, even for props that are absent (4fe4de0). This has always been the behavior in Vue 2 and is therefore considered a fix (see reasoning in #3288). However, this could break Vue 3 code that relied on the keys for prop absence checks. The workaround is to use a Symbol default value for props that need absence checks:
```
const isAbsent = Symbol()

export default {
  props: {
    foo: { default: isAbsent }
  },
  setup(props) {
    if (props.foo === isAbsent) {
      // foo is absent
    }
  }
}
```
optionMergeStrategies functions no longer receive the component instance as the 3rd argument. The argument was technically internal in Vue 2 and only used for generating warnings, and should not be needed in userland code. This removal enables much more efficient caching of option merging.

Bug Fixes

compat: fix deep data merge with extended constructor (c7efb96), closes #3852
compiler-sfc: fix style injection when using normal script + setup (8b94464), closes #3688
compiler-sfc: fix template expression assignment codegen for script setup let refs (#3626) (2c7bd42), closes #3625
runtime-core: align option merge behavior with Vue 2 (e2ca67b), closes #3566 #2791
runtime-dom/v-model: only set selectedIndex when the value changes (#3845) (ecd97ee)
suspense: fix suspense regression for errored template component (44996d1), closes #3857
watch: avoid traversing objects that are marked non-reactive (9acc9a1)
compiler-core: improve the isMemberExpression function (#3675) (9b2e894)
compiler-dom: fix in-browser attribute value decoding w/ html tags (6690372), closes #3001
compiler-sfc: correctly remove parens used for wrapping (#3582) (6bfb50a), closes #3581
reactivity: ensure computed always expose value (03a7a73), closes #3099 #910
runtime-core: fix cases of reused children arrays in render functions (#3670) (a641eb2), closes #3666
runtime-core: fix resolving inheritAttrs from mixins (#3742) (d6607c9), closes #3741
runtime-core: should disable tracking inside directive lifecycle hooks (#3699) (ff50e8d)
runtime-core: stricter compat root mount check (32e2133)
runtime-dom: should remove attribute when binding null to value (#3564) (e3f5dcb)
suspense: fix suspense patching in optimized mode (9f24195), closes #3828
transition: fix higher order transition components with merged listeners (071986a), closes #3227
keep-alive: include/exclude should work with async component (#3531) (9e3708c), closes #3529
runtime-core: properly check forwarded slots type (#3781) (e8ddf86), closes #3779
runtime-core: should not track dynamic children when the user calls a compiled slot inside template expression (#3554) (2010607), closes #3548 #3569
runtime-core/teleport: ensure the nested teleport can be unmounted correctly (#3629) (4e3f82f), closes #3623
scheduler: handle preFlush cb queued inside postFlush cb (b57e995), closes #3806
ssr: handle hydrated async component unmounted before resolve (b46a4dc), closes #3787
watch: should not leak this context to setup watch getters (1526f94), closes #3603
compat: avoid accidentally delete the modelValue prop (#3772) (4f17be7)
compat: enum coercion warning (#3755) (f01aadf)
compiler-core: fix whitespace management for slots with whitespace: 'preserve' (#3767) (47da921), closes #3766
compiler-dom: comments in the v-if branchs should be ignored when used in Transition (#3622) (7c74feb), closes #3619
compiler-sfc: support tsx in setup script (#3825) (01e8ba8), closes #3808
compiler-ssr: disable hoisting in compiler-ssr (3ef1fcc), closes #3536
devtools: send update to component owning the slot (1355ee2)
runtime-core: avoid double-setting props when casting (0255be2), closes #3371 #3384
runtime-core: avoid the proxy object polluting the slots of the internal instance (#3698) (4ce0df6), closes #3695
types: declared prop keys should always exist in props argument (#3726) (9b160b9)
types/reactivity: error TS4058 caused by RefSymbol (#2548) (90aa835)
compat: correctly merge lifecycle hooks when using Vue.extend (#3762) (2bfb8b5), closes #3761
compiler-core: bail out to array children when the element has custom directives + only one text child node (#3757) (a56ab14)
compat: handle and warn config.optionMergeStrategies (94e69fd)
compiler-core: preserve comment content in production when comments option is enabled (e486254)
hmr: don't remove __file key from component type (9db3cbb)
hydration: fix update before async component is hydrated (#3563) (c8d9683), closes #3560
reactivity: fix tracking for readonly + reactive Map (#3604) (5036c51), closes #3602
runtime-core: ensure declare prop keys are always present (4fe4de0), closes #3288
runtime-core: watching multiple sources: computed (#3066) (e7300eb), closes #3068
Teleport: avoid changing the reference of vnode.dynamicChildren (#3642) (43f7815), closes #3641
watch: avoid traversing non-plain objects (62b8f4a)
watch: this.$watch should support watching keypath (870f2a7)

yyx990803/3.1-release-note-draft.md

Features

Performance Improvements

Deprecations

Minor Breaking Changes

Bug Fixes